A MFP (multi-function peripheral) which has multiple functions including copier, printer, scanner and facsimile functions is widely used as an image processing device which performs image processing to process an input image data into an output image data. The MFP is connected to a network, receives input image data of paper documents through the scanner of the MFP, and receives other image data (document) from an external device via the communication I/F (interface) of the MFP. After the inputted images are processed through the image processing to generate output image data, the output images are printed out by the plotter of the MFP or the output image data are outputted to the external device connected with the MFP.
Conventionally, the image processing device of the above type is provided with a power saving mode in order to reduce the power consumption of the device. When an idle state of the image processing device is detected, the normal operating mode of the image processing device is changed to the power saving mode. And the image processing device returns from the power saving mode to the normal operating mode when a predetermined return condition is detected.
A control system of an image processing device which includes a sub-system which operates in the power saving mode in order to realize the power saving function is known from the patent document 1, for example. This sub-system manages the power supply of a main system which controls the whole image processing device during a normal operation, and carries out operation of the power saving mode. That is, in the power saving state, the sub-system turns off the CPU power supply of the main system, performs the network response and the monitoring of the relevant units, and, when returning from the power saving state, returns the main system to the normal operating mode again. This enables reduction of large power consumption for systems having a large CPU supply power.
In the control system of the patent document 1, an ASIC with a built-in programmable sequencer which constitutes the sub-system controls the sequences of the change to the power saving mode and the return from the power saving mode. For example, the return from the power saving mode is initiated when pressing of the operation panel switch or opening of the device cover is performed as a return trigger.    PATENT DOCUMENT 1: Japanese Laid-Open Patent Application No. 2002-268471